1. Field of the Invention
The present invention relates to an image processor and image processing method that process multi-value image data that correspond to the same area in order to print an image in the same area by relatively moving a printing unit or a plurality of printing element groups a plurality of times with respect to the same area of a printing medium.
2. Description of the Related Art
In inkjet printing devices, a multipass printing method that prints by performing a plurality of printing scans of image data to be printed in the same area of a printing medium is known as a technique for reducing density unevenness and stripes. However, recently, by performing multipass printing, a new adverse effect of image due to deviation of print position among printing scans is viewed with suspicion. Here, deviation of the printing position among printing scans is, for example, deviation between a dot group that is printed in a first printing scan and a dot group that is printed in the second printing scan in the same area as the first printing scan. Deviation of these dot groups is caused by fluctuation in the distance between the printing medium and ejection face (paper distance), fluctuation in the conveyed amount of the printing medium, and the like, when this happens, the dot coverage rate on the paper fluctuates, which causes density fluctuation and density unevenness of an image. Therefore, recently, a method for processing image data during multipass printing that is capable of handling deviation in the printing position between planes that occurs due to these kinds of fluctuations in printing conditions is desired.
In Japanese Patent Laid-Open No. 2000-103088, an image data processing method is disclosed for suppressing adverse effects to an image as described above. This patent application focuses on the cause of fluctuation in image density caused by fluctuation in various printing conditions being that the binary image data that are printed in different printing scans are in a complementary relationship with each other. In addition, in order to solve this problem, this patent application discloses a method for dividing image data in the multi-value image data stage before binarization in order to correspond to different printing scans, and then independently (uncorrelated) binarizing the multi-value image data after being divided.
FIG. 8A is a diagram that illustrates the configuration of dots when image data is divided and binarized according to the method of Japanese Patent Laid-Open No. 2000-103088. In this figure, the black dots 1501 are printed in the first printing scan, the white dots 1502 are printed in the second printing scan, and the gray dots 1503 are printed the first printing scan and second printing scan overlap. Here, there is no complementary relationship between the dots that are printed in the first printing scan and the dots that are printed in the second printing scan. Therefore, there are portions (gray dots) 1503 where two dots overlap, and there are also blank areas where no dots are printed.
In this kind of dot arrangement, the dot coverage rate on the printing medium does not fluctuate much even though the dot group that is printed in the first printing scan and the dot group that is printed in the second printing scan shift the amount of one pixel in the main scanning direction or sub scanning direction. This is because new areas where dots printed in the first printing scan overlap dots printed in the second printing scan appear, however; there are also portions where two dots that were originally supposed to overlap no longer overlap. Therefore, when performing judgment over a somewhat large area, the dot coverage on the printing medium does not fluctuate much, and thus it is also difficult for changes to occur in the image density. In other words, by employing the method of Japanese Patent Laid-Open No. 2000-103088, it is possible to suppress the occurrence of density fluctuation and density unevenness in an image even when fluctuation in the distance between the printing medium and ejection face (paper distance), or fluctuation in the conveying amount of the printing medium occurs.
However; when using the method disclosed in Japanese Patent Laid-Open No. 2000-103088, the number of pixels that are to be the object of quantization processing increases by the amount of the divided planes. Therefore, it takes more time to complete quantization when compared with the case in which quantization is performed for just one gradation plane, and thus a new problem occurs in that the processing speed becomes slower. In order to avoid this problem, a large capacity memory can be prepared, or a CPU can be prepared that has superior processing capability, however; the cost of the device then increases.